This invention relates in general to toroidal drive type continuously variable transmissions such as for use in vehicles. In particular, this invention relates to an improved apparatus and method for operating such a toroidal drive type continuously variable transmission.
In virtually all land vehicles in use today, a transmission is provided in a drive train between a source of rotational power, such as an internal combustion or diesel engine, and the driven axle and wheels of the vehicle. One common type of transmission is a discretely geared transmission, which includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such discrete gear ratios between the input shaft and the output shaft. By appropriately shifting among these various discrete gear ratios, acceleration and deceleration of the vehicle can be accomplished in a relatively smooth and efficient manner.
Another type of such transmission is a continuously variable transmission (CVT), wherein the ratio between the input shaft and the output shaft is not provided in discrete gear increments, as described above in connection with the discretely geared transmission, but rather is adjustable in a continuous or infinitely variable manner over a predetermined range. One known structure for a continuously variable transmission includes a forwardly positioned continuously variable drive section that is connected through an intermediate co-axial drive section to a rearwardly positioned output gear section. A representative structure for such a CVT is disclosed in U.S. Pat. No. 5,607,372. An electromechanical control system is often provided for controlling the operation of the CVT in a desired manner.
Traditionally, such a control system has been programmed to operate the CVT in either the torque control strategy or the ratio control strategy. When the CVT is operated in the torque control strategy, the control system correlates the throttle pedal position of the engine (as set by the driver of the vehicle using the accelerator pedal) with a desired amount of thrust for the vehicle. When the CVT is operated in the ratio control strategy, the control system controls the input speed of the CVT to a predetermined value that represents some ideal operating parameters for the engine at the current conditions. Although both of these control strategies have been effective, it has been found that both of such control strategies have disadvantages under certain operating conditions. Thus, it would be desirable to provide an improved control strategy for a CVT that achieves the advantages of both of such control strategies, while avoiding the disadvantages associated therewith.
This invention relates to an improved apparatus and method for operating a toroidal drive type continuously variable transmission (CVT). The CVT is selectively operated in either a torque control strategy and a ratio control strategy, depending upon the operating conditions of the vehicle. Thus, the CVT is operated in such a manner as to benefit from the advantageous aspects of both the torque and ratio control strategies, while avoiding the disadvantageous aspects of both strategies. The transition from the torque control strategy to the ratio control strategy (and vice versa) can be accomplished by simultaneously calculating the control valve signals that would result from operation in both the torque and ratio control strategies, and further assigning a weighted value to each of such calculated control valve signals based upon the current operating conditions. The summation of such weighted values provides a composite control signal that facilitates a smooth transition between the two control strategies. The transition from the torque control strategy to the ratio control strategy preferably occurs before a mode shift is effected. Negative feedback is provided in response to ratio changes effected by the control signals to increase stability and to compensate for sensitivity differences at different ratio angles, loading, speeds, and temperatures.